Salt of the Earth: The Source of Sodium and Chlorine in Our Oceans

As an adult, I sometimes forget to ask the obvious questions.  Kids, though, they have their heads on straight.  Living near the coast, I have the opportunity to visit the ocean pretty often.  Every once in a while I brave the frigid waters, and inevitably taste the salty sea, but I never really think about where it came from.  

Recently, I chatted with a friend about her grade school classroom.  She shared some of the science questions her students had asked.  "Why is the ocean saltier than a lake, even though it's bigger?"  I gave her the spiel: lakes and rivers have salt, but water moves through most lakes (and all rivers).  Most of that water, including salts, end up in the ocean.  Water has an easy route out of the ocean: evaporation.  Salt has no such egress.  It stays put.  Just like the ocean, some lakes and seas without outflows collect large amounts of salt. Therefore, we have Salt Lake in Utah, or the Dead Sea in Europe.  I imagined the students' next question: "Where did the salt come from in the first place?", and realized I didn't have a clue.

There are clues however.   The first is the quantity of elements in the Earth's crust.  The most common elements in sea salt vary greatly in their places on the list of most common elements in Earth's crust.  Sodium weighs in at number 6, while chlorine doesn't even make the top 20.  Sodium, therefore, is in everything.  Wikipedia lists 139 minerals, that are composed in part of the element.  When granite breaks down in water, a mineral group called feldspar releases its sodium to the water, and the water doesn't let go.  This weathering of feldspar, and other sodium minerals, would have delivered plenty of sodium to the Earth's oceans very early in their history.

Being rarer in the crust, it might seem that chlorine levels wouldn't be nearly as high in the sea.  In the ocean, chlorine content surpasses sodium as dominant element.  Why the strange ratios?  As it turns out chlorine doesn't play well with others.  A chlorine ion, which is a chlorine atom that has stolen another poor atom's electron, is large, at least relative to other common elements on Earth's surface.  The patterns these smaller ions create don't leave room for the hefty chlorine.  Chlorine elopes with a free hydrogen ion, and escapes out a volcanic vent, having never formed a rock mineral.  At the surface the hydrochloric acid splits, with the hydrogen joining oxygen to make water, and the chlorine dissolved in the ocean.  As chlorine atoms throughout geologic history jostled their way to the surface through volcanoes the oceans grew saltier.

Curiosity has always driven my study of geology.  But sometimes I forget the obvious.  The next time I visit the ocean and watch the surf gather on the shore, I'll be thinking of dissolving rock, belching volcanoes and the rivers that  bring their remnant salt downstream, my borrowed childlike wonder having been appeased.

Lorence G., Collins. "Time to Accumulate Chloride Ions in the World’s Oceans." Reports of the National Center for Science Education 26.5 (2006): n. pag. California State University Northridge. Web. 22 May 2014.

"How did the salt get into the oceans at the beginning of their formation?." UCSB Science Line sqtest. University of California, Santa Barbara, n.d. Web. 22 May 2014. <http://scienceline.ucsb.edu/getkey.php?key=2968>.

How to Make Bricks in Three Easy Steps

Last weekend my son and I foresook our weekly grocery shopping trip at the West Falmouth Hannaford.  Instead we crossed the bridge on the south end of the parking lot and ended up taking a walk on an ancient sea floor.  Don't get me wrong, this is not a magical bridge that can transport one to a fantasyland.  The bridge was built in 1859 by the railroad company to connect the Hobbs farm to the rest of society.  John Hobbs homesteaded the land in 1775, and before 1859 when the railroad isolated it and the Hobbs sold it, put it to a variety of industrious uses.  Like people of earlier times are fabled to have done, the Hobbs put every part of the land to use.  The trees became oak shingles, the soil was farmed and the clayey sediment was extracted to make bricks.  The story of these bricks extends far beyond the Hobbs enterprising spirit and thousands of years before the clay was first dug.

The story of our bricks began about 22,000 years ago when glaciers covering much of the globe decided to call it a day and begin the slow commute home from Long Island, New York (Long Island formed as the flowing ice of the glacier delivered gigantic piles of rock to its melting endpoint).  At this point, Maine and the rest of northern North America were buried in ice. Heavy, heavy ice.  The normally firm raft of rock, on which our continent sits, sank like a rowboat upon boarding.  As melting glaciers contributed to a growing ocean, this depressed plate would make way for the intrusion of a sea much larger than today's Atlantic.

The land above the sea couldn't wait to cast off its icy water.  Massive streams poured tons of melted ice into the growing sea, but the water didn't come alone. As the rivers raced to the sea their fast current picked up every stone, pebble, sand grain or mote of clay they could carry.  Massive material, like stones and pebbles may have been dropped long before reaching the sea.  When the stream met the water, it rapidly decelerated. Upon slow down, enough sand was dropped to create a river delta as deep with sand as a football field is long.  The sand from this river's mouth can still be spied along I95 between Gray and Lewiston and is currently being quarried in Gray.  But what about the bricks?

The slow moving water, at this point, would have dropped nearly everything.  Robbed of most of its energy by the trudge into the ocean, the sluggish water couldn't carry much.  Luckily, clay is small and light.  The water transported these small grains the farthest, finally dropping them at the sight of the future Hobbs' farm.  About 13,000 years ago, with the weight of ice removed, the plate rebounded, and the sea level dropped.  The acreage of the Hobbs' farm was revealed.  Soil developed.  Oaks grew.  Eventually, the Hobbs would dig up their bricks and my son's and my weight would leave footprints in the ancient seabed.  

McCully, Betsy. "Ice Age." New York Nature. N.p., n.d. Web. 11 Feb. 2014. <http://www.newyorknature.net/IceAge.html>.

Robinson, Michael A., Stewart K Sandberg, and Kirkpatrick Melissa D.,. "Using transient electromagnetic soundings to map the thickness of the Gray Delta, Maine, and correction of data using coil calibration to improve resolution. ." Geological Society of America Abstracts with Programs 33.1 (2000): 1. Print.

Weddle, Thomas K., Gray Quadrangle, Maine, 1:24,000, Augusta, ME: Natural Resources Information and Mapping, Maine Geological Survey, 1997.

"Surficial Geologic History of Maine." Maine Geological Survey. N.p., 6 Oct. 2005. Web. 11 Feb. 2014. <http://www.maine.gov/dacf/mgs/explore/surficial/facts/surficial.htm>.

"River Point Conservation Area." The Town of Falmouth . The Town of Falmouth , n.d. Web. 11 Feb. 2014. <http://www.town.falmouth.me.us/pages/falmouthme_parks/trailmaps/RiverPoint>.